1. Field of the Invention
This invention relates to color image forming devices such as color laser printers and, more particularly, to a color image forming device of which optical unit has a simplified structure.
2. Description of the Related Art
The tandem type color laser printer requires as many photoconductive drums as the colors needed for transferring, and each of the photosensitive drums must be exposed by laser. Thus the conventional color laser printer is equipped with as many scanning optical units as the photosensitive drums for exposing the respective photosensitive drums. This scanning optical unit includes components such as a polygon mirror that reflects laser light to provide a laser scanning plane, a rotation motor that rotates the polygon mirror and a reflection mirror that reflects the laser light reflected by the polygon mirror toward the photosensitive drum.
Such configuration, however, makes the resulting color laser printer expensive because it needs the installation of two or more polygon mirrors and rotation motors. Besides, since the device includes two or more scanning optical units that are similar to each other in its housing, the dimensions of the whole device grow, making it difficult to provide a compact device and easy maintenance. To solve these problems, a number of proposals have been made as follows.
FIG. 1 is a schematic diagram showing the configuration of the color image forming device disclosed in Japanese Patent Laid-Open Publication No. Hei. 10-207171. Referring now to FIG. 1, in order to make maintenance of the color image forming device easy, the exposure units 102 including polygon mirrors are collected in a specific site 104 in the housing, instead of the respective sites over the image forming units 103 including photosensitive drums. Inside the housing, a plurality of reflection mirrors 105 are provided for reflecting the laser light 101 provided by each exposure unit 102 and then directing the laser to each image formation unit 103. The image formation unit 103, which forms an image on a recording medium 106, is provided so that the recording medium 106 runs under the respective image formation units 103 one after another. In the downstream of the transfer path of the recording medium 106 beyond the image formation units 103, a fixing unit 107 is provided to fix the image created on the recording medium 106. In the device shown in FIG. 1, two or more exposure units 102 provide laser beams 101 and then two or more reflection mirrors 105 reflect the provided laser beams 101 toward the respective image formation units 103. Each image formation unit 103 forms an image on the recording medium 106, and then the fixing unit 107 fixes the formed image on the recording medium 106.
FIG. 2 is a schematic diagram showing the configuration of the color image forming device disclosed in Japanese Patent Laid-Open Publication No. Hei. 7-76137. Referring to FIG. 2, in order to reduce the quantity of polygon mirrors 108 and simplify the structure of the color image forming device, a single exposure unit 102 is provided over a position close to the center of a series of image formation units 103a-103d. The exposure unit 102 has a single rotation motor (not shown) that rotates a pair of polygon mirrors 108, and these polygon mirrors 108 expose the respective photosensitive drums. Namely, the pair of polygon mirrors 108 provide four laser beams 101 and two or more reflection mirrors 105 reflect these laser beams 101 toward the image formation units 103a-103d for image formation.
FIG. 3 is a schematic diagram showing the configuration of the color image forming device disclosed in Japanese Patent Laid-Open Publication No. Hei. 6-5409. Referring now to FIG. 3, in order to reduce the quantity of polygon mirrors 108 to one in the color image forming device of this type, a single polygon mirror 108 is provided on either side of a series of photosensitive drums 109. And, a number of reflection mirrors 105 direct the respective laser beams 101 to each photosensitive drum 109 to expose each photosensitive drum 109 are provided. The color image forming device described in Japanese Patent No. 2659360 has a configuration similar to this device.
FIG. 4 is a schematic diagram showing the configuration of the color image forming device disclosed in Japanese Patent Laid-Open Publication No. 2000-162523. Referring now to FIG. 4, the color image forming device of this type employs a single polygon mirror 108, and a plurality of reflection mirrors 105 reflect the laser beams 101 to guide them to photosensitive drums 109.
FIG. 5 is a schematic diagram showing the configuration of the color image forming device disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-305115. Referring now to FIG. 5, in order to reduce the quantity of polygon mirrors to one and eliminate reflection mirrors in the color image forming device, two or more photosensitive drums 109 are arranged around the polygon mirror 108, and the laser beams 101 provided by the polygon mirror 108 are directly guided to the respective photosensitive drums 109 without using any reflection mirrors for exposure of the photosensitive drums 109.
Such conventional devices, however, have the following problems. In the color image forming device of FIG. 1 disclosed in Japanese Patent Laid-Open Publication No. Hei. 10-207171, the resulting improvement is limited to that the exposure units 102 are collected in a specific position in the housing. It still needs a number of reflection mirrors 105 and therefore its device cost may become even higher. Even if the quantities of exposure units 102 and polygon mirrors 108 are each reduced to one, there is no change in the quantity of reflection mirrors 105, and the problem in terms of cost is left unsolved.
In the color image forming device of FIG. 2 disclosed in Japanese Patent Laid-Open Publication No. Hei. 7-76137, the laser beams 101 reflected by the polygon mirrors 108 conduct laser scanning in different directions between the image formation units 103a-103b and those 103c-103d. Then it poses a problem that the registration accuracy in the horizontal direction, which is an important factor in creating color images of high quality, becomes low. Also in this prior art device, the necessity of assembly and adjustment of such a multitude of reflection mirrors 105 leads to high costs and poor maintenance.
The color image forming devices of FIG. 3 disclosed in Japanese Patent Publication No. Hei. 6-5409 and Japanese Patent No. 2659360 use a multitude of reflection mirrors 105, as is the case with the color image forming device disclosed in Japanese Patent Laid-Open Publication No. Hei. 7-76137. Thus the device cost is raised and maintenance becomes hard to perform.
Also a multitude of reflection mirrors 105 are used in the color image forming device of FIG. 4 disclosed in Japanese Patent Laid-Open Publication No. 2000-162523, as is the case with the color image forming devices disclosed in Japanese Patent Laid-Open Publication No. Hei. 7-76137 and Japanese Patent Publication No. Hei. 6-5409. As a result, the device cost is raised, and it becomes difficult to make the device compact and maintenance easy.
Also in the color image forming device of FIG. 5 disclosed in Japanese Patent Laid-Open Publication No. Hei. 8-305115, the device becomes too large for practical use because its optical unit occupies a large installation space.
It is an object of the present invention to provide a color image forming device equipped with a simple scanning optical unit that makes the device cost low, device size compact and maintenance easy.
A color image forming device according to the present invention forms a color image of a plurality of colors onto a recording medium. This color image forming device has a laser source for emitting a plurality of laser beams based on image information, a plurality of photosensitive drums, one polygon mirror that rotates and reflects laser beams emitted from the laser sources for laser scanning, and a plurality of reflection mirrors provided for the respective photosensitive drums and located in an optical path between the polygon mirror and each of the photosensitive drums. The reflection mirrors reflect the laser beams traveling from the polygon mirror towards the photosensitive drums for scanning the respective photosensitive drums. The reflection mirror that has the longer optical path from the reflection mirror to the corresponding photosensitive drum is provided more closely to the polygon mirror than the reflection mirror having the optical path from the reflection mirror to the corresponding photosensitive drum shorter than that of the reflection mirror of longer optical path. And, all optical path lengths from the polygon mirror to the corresponding photosensitive drums via the reflection mirrors are equal to each other. Further, this color image forming device has developing units for developing latent electrostatic images formed by the laser beams on the photosensitive drums to provide an intermediate image formed by mono-color developer, an intermediate image carrier made of an endless belt and driven in one direction, the photosensitive drums which are provided along the intermediate image carrier and are in contact with and rotated with the intermediate image carrier to transfer the intermediate images of mono-color to the intermediate image carrier so as to overlap each other to form a color developer image, a transfer unit for transferring the color developer image from the intermediate image carrier to the recording medium, and a fixing unit for fixing the color developer image on the recording medium.
In the present invention, the reflection mirror having the longer optical path from the reflection mirror to the corresponding photosensitive drum is provided more closely to the polygon mirror than the reflection mirror having the optical path from the reflection mirror to the corresponding photosensitive drum shorter than that of the reflection mirror of longer optical path. Then it becomes possible to make the optical path lengths of the laser beams from the polygon mirror to each photosensitive drum via the reflection mirrors equal to each other and limit the quantity of polygon mirrors to one. In addition, it is also possible to make the number of the reflection mirrors the same as that of the photosensitive mirrors, namely, the number of colors used in images. Then the structure of the optical unit, that has the laser source, the polygon mirror and the reflection mirrors, can be simplified.
The plurality of photosensitive drums may be provided on a imaginary flat plane which makes an angle larger than zero degree and less than 90 degrees with a scanning plane of the laser beams from the polygon mirror. Then, the device can be done miniaturization and improved the precision.
Another color image forming device according to the present invention forms a color image of a plurality of colors onto a recording medium. This color image forming device has a laser source for emitting a plurality of laser beams based on image information, a plurality of photosensitive drums, one polygon mirror that rotates and reflects laser beams emitted from the laser sources for laser scanning, and a plurality of reflection mirrors which are provided for the respective photosensitive drums and located in an optical path between the polygon mirror and each of the photosensitive drums. The reflection mirrors reflect the laser beams traveling from the polygon mirror towards the photosensitive drums for scanning the respective photosensitive drums. The reflection mirror that has the longer optical path from the reflection mirror to the corresponding photosensitive drum is provided more closely to the polygon mirror than the reflection mirror that has the optical path from the reflection mirror to the corresponding photosensitive drum shorter than that of the reflection mirror of longer optical path. And, all optical path lengths from the polygon mirror to the corresponding photosensitive drums via the reflection mirrors are equal to each other. Further, this color image forming device has developing units for developing latent electrostatic images formed by the laser beams on the photosensitive drums to provide an intermediate image formed by mono-color developer, a recording medium transport unit made of an endless belt and driven in one direction, which transports the recording medium. The photosensitive drums are provided along the recording medium transport unit and are in contact with and driven with the recording medium to transfer the intermediate images of mono-color to the recording medium so as to overlap each other to form a color developer image. Further, this color image forming device has a fixing unit for fixing the color developer images on the recording medium.
In the present invention, the reflection mirror having the longer optical path from the reflection mirror to the corresponding photosensitive drum is provided more closely to the polygon mirror than the reflection mirror having the optical path from the reflection mirror to the corresponding photosensitive drum shorter than that of the reflection mirror of longer optical path. Then it becomes possible to make the optical path lengths of the laser beams from the polygon mirror to each photosensitive drum via the reflection mirrors equal to each other and limit the quantity of polygon mirrors to one. In addition, it is also possible to make the number of the reflection mirrors the same as that of the photosensitive mirrors, namely, the number of colors used in images. Then the structure of the optical unit can be simplified. Since the developer image is transferred directly from the photosensitive drums onto the recording medium, the device structure can be simplified as well.
Still another color image forming device according to the present invention forms a color image of a plurality of colors onto a recording medium. This color image forming device has a laser source for emitting a plurality of laser beams based on image information, a photosensitive belt made of an endless belt and driven in one direction, on which the laser beams are exposed at a plurality of laser beam exposure portions different from each other and provided along on the photosensitive belt, and a plurality pairs of an electrostatic charger and a developing roller provided for each of the laser beam exposure portions. The electrostatic charger charges the photosensitive belt and the developing roller develops a latent image formed by exposure of the laser beam. This color image forming device has further, one polygon mirror that rotates and reflects laser beams emitted from the laser sources for laser scanning, and a plurality of reflection mirrors which reflects the laser beams traveling from the polygon mirror towards the photosensitive belt for scanning the photosensitive belt at the laser beam exposure portions, to form the latent images so as to form intermediate images of mono-color developer by the developing roller on the photosensitive belt driven in one direction so as to overlap each other to form a color developer image. The reflection mirror that has the longer optical path from the reflection mirror to the corresponding laser beam exposure portion is provided more closely to the polygon mirror than the reflection mirror that has the optical path from the reflection mirror to the corresponding exposure portion shorter than that of the reflection mirror of longer optical path. And, all optical path lengths from the polygon mirror to the corresponding exposure portions via the reflection mirrors are equal to each other. Further, this color image forming device has a transfer unit for transferring the color developer image from the photosensitive belt to the recording medium, and a fixing unit for fixing the color developer images on the recording medium.
In the present invention, the reflection mirror that has the longer optical path from the reflection mirror to the corresponding exposure portion is provided more closely to the polygon mirror than the reflection mirror that has the optical path from the reflection mirror to the corresponding exposure portion shorter than that of the reflection mirror of longer optical path. Then it becomes possible to make the optical path lengths of the laser beams from the polygon mirror to each exposure portions on the photosensitive belt via the reflection mirrors equal to each other and limit the quantity of polygon mirrors to one. In addition, it is also possible to make the number of the reflection mirrors the same as that of the exposure portions, namely, the number of colors used in images. Then the structure of the optical unit can be simplified. In addition, the device can be made compact and inexpensive because the quantity of photoreceptors is limited to one by adopting the photosensitive belt made of an endless belt.
The endless belt, namely, intermediate image carrier, recording medium transport unit and photosensitive belt may be provided more closely to the polygon mirror than the fixing unit. Then the influence of heat generated in the fixing unit upon the polygon mirror can be weakened.
As described above, since the color image forming device according to the present invention has a simplified scanning optical unit, it can be cost-effective, compact and easy for maintenance. This invention can be applied to the color image forming device whether it employs a method of using an intermediate belt or a method of transferring images directly onto the recording medium.